This invention relates to a controller such as a proportional plus integral, or proportional plus integral plus differential, controller for controlling a controlled system in connection with detecting means for detection of the output of the controlled system to provide a corresponding feedback signal to the controller.
Conventional process or other type control systems include a controller of the type mentioned above. If consideration is given to such a control system which is composed of a proportional plus integral action controller associated with a controlled system and detecting means which detects the output or controlled variable of the controlled system and imparts a corresponding feedback signal to the controller, the output or controlled variable of the controlled system will generally exhibit an decreasing oscillatory overshooting response to a step input signal to the controller. It is pointed out that such decreasing oscillatory response is due mainly to the proportional plus integral action of the controller. That is, after the application of the step input signal, the controller will perform the proportional and integral action on the error input as the difference between the step input signal and the feedback signal. Thus, even when the error or proportional input becomes zero, that is, even when the output of the controlled system reaches its desired value, the integrated output component of the controller remains thereby causing an overshoot. After the occurrence of the overshooting, the error is inverted in sign and the overshoot increases until the integrated output component and the error become equal to and are cancelled by each other.